No Sex Bias in Wood Thrushes (Hylocichla mustelina) Captured by Using Audio Playback during the Non-breeding Season
2014
Conservation of migratory songbird species requires information on abundance and survival over the annual cycle, including from overwintering sites. Broadcasting recorded calls or songs (playback) often increases detections or capture rates of birds, and can improve estimates of abundance or survival. Wood Thrushes overwintering in Belize regularly respond to broadcasted conspecific vocalizations (songs); however, it is unknown if song attracts a specific age, sex, or size class of birds. Our goal was to determine if the use of playback resulted in sex, age, or size-biases in captures of Wood Thrushes, relative to captures without playback, at a non-breeding site in Belize, Central America. We predicted that birds responding to playback would be: (1) male, (2) adults, and (3) larger than birds caught without playback, owing to social dominance of larger adult males over juveniles and females. Surprisingly, we found no significant difference in sex or age ratio, or body size of birds captured with or without playback. It may be that predicted patterns of social dominance are not apparent in non-breeding Wood Thrushes, or that song playback does not elicit responses related to local social dominance. Regardless of the mechanism, we conclude that the use of song playback during the non-breeding season does not bias captures of Wood Thrushes by sex, age, or body size, and when used with caution, can be an effective tool for studies on the species' wintering grounds.
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